The present invention relates to improvements in color printers of the type which employ an asynchronous laser scanner or the like to sequentially record a series of color-separated images on a recording element moving along a path. More particularly, it relates to apparatus for reducing registration errors between successively formed color-separated images.
In the commonly assigned U.S. Patent Application Ser. No. 688,004, filed on Apr. 19, 1991 in the name of Kevin M. Johnson and entitled POSITION CONTROL APPARATUS FOR TRANSFER DRUM IN ELECTROSTATOGRAPHIC PRINTER/COPIER, there is disclosed a color electrophotographic printer for producing multicolor prints on image receiver sheets. Such a printer comprises a drum-shaped recording element which is rotatably driven about its axis to present its photoconductive outer surface to the various processing stations required to produce transferable color-separated images thereon. During each revolution of the photoconductive drum, one complete color-separated image is formed. This image is immediately transferred from the outer surface of the image process drum to a receiver sheet carried on the outer surface of a rotating transfer drum as the respective drum surfaces pass through a transfer zone defined by a nip between the drums. The process drum is then recycled through the processing stations to receive additional color-separated images which, when transferred to the receiver sheet in registration with previously transferred color-separated images, provide a full color print of a desired image. Obviously, the ultimate print quality depends, in large part, on the sharpness in the registration among the transferred images, and such registration depends, in part, on the ability to produce transferable images at a precise location on the photoconductive surface of the recording element.
In the color printer described above, a raster image scanner, in this case a conventional laser scanner, is used to imagewise expose the photoconductive drum in order to produce a developable, latent (i.e. electrostatic) image thereon. In producing this image, the laser scanner repeatedly scans the moving surface of the photoconductive drum with an intensity-modulated beam of actinic radiation, whereby the image is produced on a line-by-line basis. Scanning of the beam is effected by a rotating polygon (or hologon) having a plurality of reflective facets which interact with the laser beam, one at a time, to sweep the beam across the photoconductive surface. The polygon rotates asynchronously with respect to the rotation of the photoconductive drum, meaning that the position in which the laser scanner records an image is not precisely linked with the drum position.
In the above apparatus, relatively accurate placement of the image on the surface of the photoconductive drum is achieved by detecting a mark on the drum perimeter. The output of the mark detector provides a control signal to a logic and control unit (LCU) to indicate that the beginning of the image frame on the photoconductive drum is approaching the exposure station. After an appropriate delay, the LCU directs the laser scanner to start the image exposure. If a polygon facet happens to be located such that it is directing the laser beam at the start of a scan line at the exact instant that the LCU commands that imaging begin, there will be no error in the position of the image vis-a-vis a nominal position on the drum surface. However, since the scanner operates asynchronously with respect to the movement of the drum, this is rarely the case. Most often, none of the reflective facets of the rotating polygon will be in a position to start a scan line at the precise instant that the LCU directs that printing should begin. Whenever the polygon must be further rotated to properly position one of its facets to start a new scan line following the LCU's print command, there will be a slight delay in the start of the image formation. This delay translates into a slight displacement error (e.g., by as much as 50 microns, in the case where the number of scan lines per centimeter is 200) of the position of the latent image relative to a nominal position along the perimeter of the drum. Thus, it may be appreciated that each color separated image on the drum may be misregistered with respect to previously formed images by as much as the width of one image line, depending on the rotational position of the polygon when the print command signal is produced. It will be noted that this misregistration is always biased in one direction. That is, the error is between zero and one line; there is no negative error.
The above-noted misregistration of the color-separated images on the photoconductive recording element may be compensated for by controlling the rate of advancement of the receiver sheets to the image transfer zone, slowing down the sheets when the image is formed too late vis-a-vis a nominal time, and speeding up the sheets when the image is formed too early. Such a compensation scheme is disclosed in the commonly assigned U.S. Pat. No. 4,893,135 issued in the name of Fereidoon S. Jamzadeh which discloses an asynchronously operated laser printer which electrophotographically records color-separated images on an endless photoconductive web. However, rather than providing a means for compensating for image misregistration after it has occurred, it is always desirable to minimize the error for which compensation is required. The present invention addresses the task of minimizing the above-noted image registration error on the recording element.